Transistor with self-aligned terminal contacts

ABSTRACT

An embodiment of a MOS transistor includes a layer of semiconductor material, drain regions having a first conductivity type alternately formed in the layer with body regions having a second conductivity type, a first insulating layer disposed over the surface of the layer of semiconductor material, at least one gate-precursor region of conductive material disposed over the first insulating layer, a second insulating layer disposed over the first insulating layer and the gate-precursor region, a third insulating layer disposed over the second insulating layer, at least one source opening formed by removing overlapping portions of the second insulating layer, the third insulating layer, the gate-precursor region, and by at least partially removing a corresponding portion of the first insulating layer. The embodiment may also include at least one source-precursor region extending into the layer of semiconductor material from a surface portion below the at least one source opening.

PRIORITY CLAIM

The instant application claims priority to Italian Patent Application No. MI2012A001244, filed 17 Jul. 2012, which application is incorporated herein by reference in its entirety.

SUMMARY

An embodiment generally relates to semiconductor devices. In more detail, an embodiment refers to field-effect transistors.

Recently, the growing trend of the integration-density increase of semiconductor devices has led to the size reduction of the elements used in integrated circuits (a process often called “scaling”), to allow the realization of complete integrated electronic systems. In particular, electronic systems including one or more circuits for management and distribution of electrical energy (often called “power circuits”) next to the signal-processing circuits have had a great development, as in the case of the BCD (Bipolar CMOS DMOS) technology.

A basic integrated-circuit element is the transistor; in particular, in circuits with a high integration density, field-effect transistors are used, and in particular, transistors of the MOS type. Moreover, the integrated MOS transistors forming the integrated power circuit (as in motors-driving circuits and the like) in addition to being formed with small dimensions, are also able to withstand relatively high voltages (e.g., 10 Volts (V)-70V) and supply high currents (e.g., 1 Ampere (A)-10 A) to loads.

Unfortunately, the limitations in the realization of MOS transistors of small dimensions often arise from the limits of the technological processes used in the realization thereof. Indeed, the size reduction of the transistors (and more generally, of each electronic device) is hindered by constraints to which current integration techniques are subject to the voltages borne.

As described in U.S. Patent Applications 2011/0012267 and 2011/0018068, which are incorporated herein by reference, in the case of integrated MOS power transistors, it is known to form contact regions between regions and terminals of such an integrated power MOS on planes at different heights and close to each other in order to increase the compactness of the transistor.

However, a limit to the size reduction is imposed by the formation of an opening—called a source opening—for the definition of source regions and a body contact (for electrically coupling to a body region) of the MOS transistor by means of photolithography processes.

The optical lithography or photolithography, typically uses a projection of electromagnetic radiation through a mask for defining regions in semiconductor materials, conductive materials, or insulating materials to form one or more electronic devices (e.g., transistors) in a chip of semiconductor material.

In the MOS transistor, the source opening is formed on a plane at a different height (e.g., lower) relative to a plane (corresponding to the focal plane of the mask) on which are formed other portions of the transistor itself (such as drain contact regions and part of a gate electrode) and portions of low-voltage transistors for processing the digital signals generated on the same chip (in the case of BCD electronic systems).

Therefore, the source regions and the body contact (defined through a plurality of photolithographic masks) may be heavily affected by optical aberrations. In fact, the optical aberrations are accentuated in the case where the plane on which is defined a region is not located on the focal plane of the mask (i.e., are out of focus), as in the case just described. Consequently, the areas outside the focal plane of the mask may be defined in an imprecise manner (in particular the edges of the regions) inversely proportional to the size of such regions (due to the optical aberrations). In other words, there may be a reduced control over the size of the regions defined by the mask.

Generally, to compensate for optical distortion, one may increase the size of the photolithographic mask, or increase tolerances in the distances between regions to be defined. Unfortunately, this may have the effect of increasing the overall dimensions of the transistor (with respect to the case in which a surface of the chip is uniform), in other words, the reduction in the size of the transistor may be limited by the tolerances used to correct the optical deformations mentioned above.

In general terms, an embodiment is based on the idea of defining compact source regions and the body contact through a self-alignment process.

More specifically, an embodiment is a method for integrating a MOS transistor on a chip of semiconductor material. The method includes providing a chip structure that includes a layer of semiconductor material, in which are alternately formed drain regions with a first conductivity type and body regions with a second conductivity type, a first insulating layer disposed over the surface of the substrate, at least one gate-precursor region of conductive material disposed over the first insulating layer, a second insulating layer disposed over the first insulating layer and the gate-precursor region, and a third insulating layer disposed over the second insulating layer. The method further includes forming at least one source opening by removing overlapping portions, of the second insulating layer, the third insulating layer, the gate-precursor region and at least partially removing a corresponding portion of the first insulating layer. In an embodiment, the method further includes forming at least one source-precursor region extending into the substrate from a surface portion below the at least one source opening, forming at least one fourth insulating layer on walls of the first insulating layer, the second insulating layer, the third insulating layer, and the gate-precursor region facing the source opening, prolonging the at least one source opening inside the substrate by removing a portion of the source-precursor region and exposing a further portion of the substrate, the step of extending the at least one source opening inside the substrate defining at least two source regions separated by the source opening, and forming at least one body contact region extending into the substrate from the further portion of the substrate exposed from the at least one source opening.

Another embodiment provides a corresponding integrated MOS transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments, as well as features and advantages thereof, will be better understood with reference to the following detailed description, given purely by way of a non-restrictive indication and without limitation, to be read in conjunction with the attached figures (wherein corresponding elements are denoted with equal or similar references and their explanation is not repeated for the sake of brevity). In this respect, it is expressly understood that the figures are not necessarily drawn to scale (with some details that may be exaggerated or simplified) and that, unless otherwise specified, they are simply intended to conceptually illustrate the structures and procedures described herein. In particular:

FIG. 1 is a schematic cross-sectional view of a portion of an MOS transistor, according to an embodiment.

FIG. 2A-21 show some phases of a formation process of a MOS transistor, such as the MOS transistor of FIG. 1, according to an embodiment.

DETAILED DESCRIPTION

With reference to the figures, FIG. 1 is a schematic cross-sectional view of a portion of transistor 100, according to an embodiment.

The transistor 100 is integrated in a chip which includes a layer indicated as a whole by reference number 105 of semiconductor material where alternate regions are made, each with a doping complementary to the doping of the adjacent regions. In the example considered, a drain region 102 of N-type (as in the case of Silicon doped with Phosphorus, Arsenic or Antimony) and a body region 104 of P-type (as in the case of Silicon doped with Boron) are formed. Moreover, the transistor 100 has a cellular structure, including a plurality of unit cells 110 (of which only one cell 110 and portions of a cell to the left and of a cell to the right are visible in the figures). The unit cells 110 are arranged in one or more sequences, each including at least one pair of cells 110, in which each cell 110 is formed in a mirrored way with respect to each adjacent cell 110 in the sequence.

The cellular structure allows obtaining a flow of electric current of high intensity without an excessive overheating of the transistor 100 during operation thereof—by intercoupling to one another corresponding terminals of each cell 110 (because the flow of electric current is divided between the cells 110 of the cell structure).

Each unit cell 110 forms a single transistor for example—of the MOS type—including a drain contact region 115 formed in an upper portion of the drain region 102. In particular, the drain contact region 115 is of the N-type as the drain region 102, but is characterized by a greater doping than the same (i.e., in the drain contact region 115 is present a greater number of charge carriers than in the surrounding drain region 102).

In addition, each unit cell 110 includes a gate region 120 of conductive material—for example, polycrystalline silicon or polysilicon—and a source region 125—of the N-type—and a body contact region 130—of the P-type. In particular, the source region 125 and the body contact region 130 are formed in an upper portion of the body region 104, with the body contact region 130 having a doping level greater than the doping level of the body region 104.

In addition, the body region 104 and the body contact region 130 are shared by a pair of cells 110 formed adjacent including a cell 110 in an odd position in the sequence and a subsequent cell 110 in an even position in the sequence or, vice-versa, from a pair of cells 110 adjacent-formed including a cell 110 in an even position in the sequence and a subsequent cell 110 in an odd position in the sequence (formed substantially symmetrical with respect to the body region 104 and the body contact region 130). Similarly, the drain region 102 and the drain contact region 115 are shared by an alternative pair of cells 110 adjacent-formed including a cell 110 in an even position in the sequence and a previous cell 110 in an odd position in the sequence or, vice-versa, from a pair of cells 110 adjacent-formed including a cell 110 in an odd position in the sequence and a previous cell 110 in an even position in the sequence (formed substantially symmetrical with respect to the drain region 102 and the drain contact region 130).

In detail, the drain contact region 115, is formed in the drain region 102, while the source regions 125 and the body contact 130 are formed in the body region 104, included in the layer 105 in correspondence of a surface 105 a thereof (as will be described in greater detail in the following). It is noted that a first height y1 of the surface 105 a where the drain contact region 115 is formed is higher than a second height y2 of the surface 105 a where the source regions 125 and the body contact 130 are formed; this allows that, during the operation of the transistor 100, a channel region (in a portion of the body region 104 below the gate electrode) and a drain extension sufficiently long, at the same time containing a lateral extension x/of each cell 110 (as known).

On the surface 105 a of the substrate 105 a first (field) insulating layer 135—for example, Silicon Oxide—is disposed to substantially isolate the substrate 105 from the gate region 120. Indeed, above the insulating layer 135, substantially between the drain contact region 115 and the source regions 125 and the body contact 130 the gate region 120 is disposed.

In detail, the first insulating layer 135 has a profile of maximum thickness between the drain contact region 115 and a central portion of the gate region 120 that tapers until it reaches a minimum thickness in the proximity of the source region 125. The gate region 120 then follows the profile of the first insulating layer 135 but is formed with a substantially constant thickness. The profile and the thickness of the first insulating layer can be designed to optimize (during the operation of the transistor 100) a pattern of an electromagnetic field generated in the substrate below the gate region 120 by means of a biasing of the latter.

Over the first insulating layer 135 and the gate region 120, are arranged a second (internal) insulating layer 140 for example, silicon nitride and a third (surface) insulating layer 145 (for example, with a high degree of resistance to ion penetration)—for example, Silicon Oxide doped with Phosphorus to electrically and physically isolate the cell 105 from the upper layers such as, for example, metallization layers (not shown in the figures).

To realize the electrical contacts for biasing (during operation) the contact regions of drain 115, gate 120, source 125, and body 130 are provided corresponding drain 150, gate 155, source/body 160 contacts of conductor material—for example, tungsten—that pass through the insulating layers 135, 140 and 145 reaching such regions.

In an embodiment, the body contact region 130 is provided at a depth y3 from the surface 105 a lower than the source region 125 which is formed substantially at the surface 105 a. In addition, the body contact region 130 (adapted to bias the body region 104 during operation of the transistor 100) is formed with a reduced body lateral extension x2 (e.g., lower than approximately 0.8 μm, for example, lower than approximately 0.6 μm, such as approximately 0.4 μm). In addition, the source/body contact 160 is surrounded by a vertical insulating layer 165—for example, again in Silicon Oxide—, which isolates the source/body contact 160 from the adjacent gate region 120. In this way, the source regions 125 and the body contact 130 are electrically intercoupled with each other (so as to operate at the same potential during operation of the transistor 100). In addition, the arrangement of the body contact region 130 adjacent to the source region 125, but formed at the depth y3 allows providing a source/body contact 160 with a reduced contact lateral extension x3 (e.g., equal to or lower than the body lateral extension x2). Indeed, the source regions 125 are contacted through the side portions of the source/body contact 160, while the body contact region 104 is coupled to an end portion thereof (and possibly also to the side portions thereof).

The body lateral extension x2 and the reduced contact lateral extension x3 of the body contact region 130 and of the source/body contact 160 also imply that the cells 110 of the sequence have a proportionally reduced lateral extension x1; therefore, the integrated transistor 100 will occupy a correspondingly reduced portion of the chip of semiconductor material. Therefore, a further portion of the chip of semiconductor material may be available for integrating additional electronic components, such as logic circuits and/or other power electronic components. In other words, the use of the transistor 100 according to an embodiment allows obtaining a denser integration of electronic devices on the same chip.

It should be noted that the transistor 100 just described is designed to operate at relatively high voltage intervals (e.g., at voltages range between approximately 5V and V, for example approximately between 8V and 70V, such as approximately between 10V and 50V) and suitable to be integrated onto a same chip in which are also integrated electronic devices (for example, additional transistors) designed to operate at lower voltage ranges (for example, at voltages approximately between 5V and 0V, for example approximately between 3V and 0V, such as approximately between 1V and 0V).

Considering now FIGS. 2A-2H, jointly, a process of forming a MOS transistor according to an embodiment is described, of which the figures show some steps.

With particular reference to FIG. 2A, the process is described starting from a structure including the substrate 105, the drain contact region 115, the first and the second and the third insulating layers 135, 140 and 145 (formed in a known manner and therefore not described in detail). In particular, the structure also includes a gate-precursor region 202, which is substantially equally shared between alternating pairs of adjacent cells 110.

In this phase, a source mask 205 is arranged onto the structure at a surface 145 a of the insulating layer 145 (i.e. on the top of the structure). The source mask 205 may include a Bottom Anti-Reflective Coating or BARC (not shown in the figures) adapted to reduce optical distortion of a photolithographic process due to reflections at the surface 145 a.

Turning now to FIG. 2B, the structure is subjected to a first anisotropic etching (conceptually indicated by an arrow in the figure, for example, a plasma etching as the RIE technique, Reactive Ion Etching) to form a source opening 210 having a lateral extension x3. In particular, the opening 210 is formed by removing a portion of the insulating layers 140 and 145, of the precursor region of gate 202 and at least partially a portion of the insulating layer 135 determined by the mask 205. In addition, the first anisotropic etching defines the two gate regions 120 of the pairs of adjacent cells 110.

As shown in FIG. 2C, a source-precursor region 215 is formed in the substrate 105 from the surface 105 a thereof. The source-precursor region 215 may be formed thanks to a phase of ion implantation (conceptually represented by a plurality of arrows). A partial removal of the insulating layer 135 reduces structural damages to the body region 104 due to the impact force of the ions during their implantation. Indeed, the insulating layer 135 reduces the impact force of the ions without reducing the efficacy of the ion-implantation phase, thus allowing forming a more robust and efficient transistor 100.

With reference to FIG. 2D, the source mask 205 is removed from the structure (exposing the surface 145 a of the insulating layer 145). Then, an insulating material 220 (e.g., Silicon Oxide) is deposited over the entire exposed surface and penetrates inside the source opening 210 (for example, by a conformal deposition in vapor phase). The deposition takes place on the walls of the insulating layers 135, 140, and 145 and the gate region 120, which delimit the source opening 210. In particular, with this deposition phase the gate region 120 is electrically isolated from the source opening 210. In addition, a lateral extension of the source opening 210 is reduced proportionally to the insulating material 220 deposited (for example, the lateral extension of the source opening 210 is reduced compared to the initial value to approximately 0.1 to 0.7, for example is reduced to approximately 0.2 to 0.5, and such as reduced to approximately 0.25 to 0.4, as a reduction of approximately 0.3 times compared to the initial value of the lateral extension). The insulating material 220 may be deposited through a vapor-phase deposition (thus, the deposition of the insulating material 220 may occur rapidly and economically).

Subsequently, an excess of insulating material 220 is removed from the surface 145 a of the insulating layer 145; for example, by a step of chemical mechanical planarization or CMP (Chemical-Mechanical planarization) then defining the vertical insulating layer 165.

Turning now to FIG. 2E, the structure is subjected to a second anisotropic etching (conceptually shown in the figure by another arrow; for example, again a RIE technique) substantially orthogonal to the surface portion 105 a of substrate 105 exposed on the bottom of the source opening 210. In this way, the second anisotropic etching has the effect of extending the source opening 210 within the substrate 105 in the body region 104. In particular, the second anisotropic etching removes a (central) portion of the source-precursor region 215 (substantially shared by two adjacent cells 110). Therefore, at the same time two source regions 125 are defined belonging to two adjacent cells 110 (substantially symmetrical with respect to the source opening 210).

It is noted that the insulating layers 145 and 165 compensate for the absence of a mask at this phase; indeed, the second anisotropic etching has an effect only on the source-precursor region 215 and on the substrate region 105 on the bottom of the source opening 210. In contrast, the remaining structure is shielded by the insulating layers 145 and 165 and is not subjected to the second etching.

Considering now FIG. 2F, the body contact region 130 is formed in the substrate 105 from the portion thereof which delimits the source opening 210. The body contact region 130 may be formed thanks to an ion-implantation phase (represented conceptually by another plurality of arrows in the figure).

In particular, the implantation phase of the body contact region 130 is performed without the need for arranging a special mask on the structure. As in the previous phase, the insulating layers 145 and 165 compensate for the absence of the mask by shielding regions of the structure from the implantation of semiconductor material. In addition, a lateral extension of the insulating layer vertical 165 (defined by the thickness deposited in the previous deposition phase thereof) substantially determines the lateral extension x2 of the body contact region 130 (resulting in the reduction of the lateral extension of the source opening 210).

This implies that the implantation phase of the body contact region 130 in the portion of substrate 105 that delimits one end of the source opening 210 is not subject to optical distortion mentioned above; accordingly, it is not necessary to impose significant tolerances to distances between regions to be defined, and may be obtained by the body regions 130 with the body lateral extension x2 contained (e.g., equal to approximately 0.9 to 0.1, for example, approximately 0.5 times the body lateral extension x2 that would be obtained by using a mask). In addition, the ion-implantation phase, substantially, introduces few, if any, impurities (i.e., unwanted particles with an unsuitable atomic number, for example Boron atoms in an N region) and causes little or no weakness in the structure of active regions (for example, the drain 115, gate 120 and source 125 regions) of the structure.

Turning now to FIG. 2G, on the structure is arranged a gate and drain contact mask 230 in correspondence of a surface 145 a of the third insulating layer 145 (i.e. on the top of the structure). The gate and drain contact mask 230 allows defining corresponding gate 235 and drain 240 openings. Also in this case, the gate and drain contact mask 230 may including an anti-reflection sub-layer. Furthermore, this gate and drain contact mask 230 may coincide with a mask used to define contacts of other electronic devices present in the same chip (both suitable for high voltage levels and suitable for voltage levels reduced as the case of an integrated system in BCD technology). Subsequently, the structure is subjected to a third anisotropic etching that removes portions of the insulating layers 140 and 145 down to the point of exposing respective portions of the gate region 120 and of the drain region 115.

With reference to FIG. 2H, the gate and drain contact mask 230 is removed and there is a phase of deposition of conductive material 245 (typically including a combination of different materials; for example, Titanium, Titanium Nitride, and Tungsten) suitable for ensuring a good electrical contact with the exposed surfaces of the regions 115, 120, 125, and 130 (for example silicon or silicide of refractory materials) and suitable to ensure a good filling of the openings 210, 235, and 240. The total amount deposited is such as to completely fill the source opening 210, the gate opening 235 and the drain opening 240 (i.e., from the corresponding source regions 125 and the body 130, gate 120 and drain 115 contact up to the 145 a surface of the third insulating layer 145).

Turning now to FIG. 2I, the structure is subjected to a further phase of chemical-mechanical planarization in order to remove excess conductive material 245 and even out the surface 145 a of the insulating layer 145 followed by a heat treatment to ensure good contact between the source region and the body contact. Consequently, the source/body contact 160, the gate contact 155, and the drain contact 150 are completely defined.

At the end of this phase, the above-described transistor 100 is formed. The source/body contact 160, the gate contact 155, and the drain contact 150 are exposed on the surface 145 a of the insulating layer 145 in such a way as to allow forming an electrical coupling of the corresponding source 125 and body 104 regions (through the body contact region 130), the gate 125 and the drain 102 (through the drain contact region 115) with the upper layers (for example, by metallization layers, not shown in the figures) or other electronic components integrated in the chip of semiconductor material.

It is noted that the process described above may require only one additional photolithographic mask with respect to a traditional process to define the source opening 210; therefore, the transistor 100 is subject to inaccuracies due to optical aberrations to a lesser extent compared to transistors made using traditional techniques.

The steps described above may be executed in parallel, i.e., at the same time, on more unit cells 110 included in the transistor 100. In addition, the steps described above may be performed in parallel on multiple cells 110 belonging to more transistors 100. In general, alternative embodiments may be implemented using one or more equivalent methods (using similar steps with the same functions of several steps or portions thereof, by removing some non-essential steps, or by adding additional optional steps); also the steps may be performed in different order, in parallel or overlapping (at least in part). Furthermore, the source region 125 of a unit cell 110 may be a single region having an opening through which the source/body contact 160 extends, instead of the source/body contact dividing the source region into two separate regions 125. For example, referring to FIG. 1, the visible source-region portions 125 may form a single source region 125 by joining together in a plane other than the plane of FIG. 1.

Finally, it is noted that although in the description reference has been made only to an N-type transistor, it is possible to provide an embodiment including a complementary transistor of the P-type (i.e., with P-type drain regions and drain contact and source and body regions and the body contact of the N-type).

An integrated circuit including the transistor 100 may be coupled to at least one other integrated circuit to form a system. For example, at least one of these integrated circuits may be a computing circuit such as a microprocessor or microcontroller, and these integrated circuits may be disposed on the same, or on different, integrated-circuit dies. And examples of such a system include a smart phone, a camera, and a computing device such as a desktop, laptop, pad, or handheld computing device.

From the foregoing it will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the disclosure. Furthermore, where an alternative is disclosed for a particular embodiment, this alternative may also apply to other embodiments even if not specifically stated. 

1-10. (canceled)
 11. A transistor, comprising: a drain region; a body region disposed adjacent to the drain region; a first source region disposed in the body region; a body-contact region disposed in the body region beneath the source region; and a source-body contact having a side in contact with the source region and having a bottom in contact with the body-contact region.
 12. The transistor of claim 11 wherein the body region is disposed in the drain region.
 13. The transistor of claim 11 wherein: the drain and source regions are of a first conductivity type; and the body and body-contact regions are of a second conductivity type.
 14. The transistor of claim 11, further comprising: an opening that extends through the source region and into the body-contact region; and the body contact is disposed in the opening.
 15. The transistor of claim 11, further comprising: a second source region disposed in the body region over the body-contact region; and wherein the source-body contact is disposed between the first and second source regions and has a side in contact with the second source region.
 16. The transistor of claim 15 wherein the source-body contact has a same side in contact with the first and second source regions.
 17. The transistor of claim 15 wherein the source-body contact has respective sides in contact with the first and second source regions.
 18. The transistor of claim 11 wherein the body contact region is in contact with a side of the source-body contact.
 19. The transistor of claim 11, further comprising: a gate insulator disposed over a portion of the body region disposed between the drain region and the source region; and a gate disposed over the gate insulator.
 20. An integrated circuit, comprising: a layer of semiconductor material; a drain region disposed in the layer; a body region disposed in the layer adjacent to the drain region; a source region disposed in the body region; a body-contact region disposed in the body region beneath the source region; and a source-body contact having a side in contact with the source region and having a bottom in contact with the body-contact region.
 21. The integrated circuit of claim 20, further comprising: a gate insulator disposed over a portion of the layer disposed between the drain region and the source region; and a gate disposed over the gate insulator.
 22. A system, comprising; a first integrated circuit, including a layer of semiconductor material, a drain region disposed in the layer, a body region disposed in the layer adjacent to the drain region, a source region disposed in the body region, a body-contact region disposed in the body region beneath the source region, and a source-body contact having a side in contact with the source region and having a bottom in contact with the body-contact region; and a second integrated circuit coupled to the first integrated circuit.
 23. The system of claim 22 wherein the first and second integrated circuits are disposed on a same die.
 24. The system of claim 22 wherein the first and second integrated circuits are disposed on respective dies.
 25. The system of claim 22 wherein at least one of the first and second integrated circuits includes a computing circuit.
 26. A method, comprising: forming through a first opening above a body region a source region in the body region; forming through the first opening a second opening in the body region adjacent to the source region; forming through the second opening a body-contact region in the body region; and forming a source-body contact in the second opening.
 27. The method of claim 26, further comprising forming the first opening in a layer that is disposed above the body region;
 28. The method of claim 26 wherein forming the second opening includes forming the second opening through the source region.
 29. The method of claim 26 wherein forming the second opening includes dividing the source region into first and second separate source regions with the second opening.
 30. The method of claim 26 wherein forming the second opening includes: forming a layer over a side and a bottom of the first opening to form an intermediate opening; and forming the second opening by removing through the intermediate opening a portion of the layer disposed over the bottom of the first opening. 